It is common practice for refineries to dispose of combustible hydrocarbon containing waste gases by burning the waste gas in a flare. Refineries built to process combustible gases are designed so that the vessels used in the refining process are constructed to withstand normal pressure variations during routine plant operations. However, to prevent rupture of these vessels when actual operating conditions are such that pressures exceed their design pressure limits, safety relief valves are placed on these vessels to vent the high pressure gases. These vented gases can be passed to a flare system where the combustible gases are burned. The high pressure waste gas flow vented to a flare is a primary component of the flare load.
While it is within the capability of one skilled in the art to design a flare stack that will burn combustible gases, such designs generally require that the amount and composition of the waste gases be reasonably constant. In actual refining operations, however, the flow rate of waste gas to a flare is generally not constant, and thus it is often necessary to reduce the flame size and corresponding heat emission from a flare that is highly loaded with waste gases being vented to that flare.
In order to reduce the flame size and/or heat emission from a highly loaded flare stack, it has been proposed to divert a portion of the flow of the waste gas from a highly loaded flare stack to an alternate parallel flare stack that is lightly loaded; however, a simple crossover conduit to connect a heavily loaded flare stack to a lightly loaded flare stack is impractical because an unsealed crossover conduit could result in one flare burner drafting the other flare stack during periods of low flow, thereby creating a backflow by sucking the flames and air back into a flare stack conduit.
In the past it has been proposed to eliminate the backflow condition created by an unsealed crossover conduit between two parallel arranged flare stacks by utilizing pressure controllers which manipulate a butterfly valve in the crossover conduit; however, the control method which manipulates a butterfly valve in the crossover conduit is subject to certain limitations. For example, the waste gas flowing in the crossover conduit could contain contaminants which would foul operation of a butterfly valve and render the utilization of a butterfly valve unreliable. Further, regardless of the contaminants in the waste gas being flared, butterfly valves, especially when used in large sized conduits, are generally unreliable as final control elements. For one example, a butterfly valve requires unequal torque for opening and for closing.
Accordingly, it is an object of this invention to provide improved method and apparatus for diverting a portion of the high pressure waste gas being vented to a flare to an alternate parallel flare if the load of waste gas exceeds a maximum desired load.
It is another object of this invention to provide improved flaring of waste gases by employing liquid seal tanks which are connected to act as valves to effect the diversion of waste gases to an alternate parallel flare stack during periods of high load and which automatically reseals on return to normal loads.
It is a further object of this invention to share a load of waste gas between two parallel arranged flares during periods of high process relief.
It is yet another object of this invention to provide method and apparatus for the venting of waste gases to flare stacks which are safe, economical and reliable.